Not Applicable.
1. Field of Invention
The present invention relates generally to optics and, more particularly, to systems and methods for measuring the spot size of a focused optical spot in either a far-field or near-field system.
2. Description of the Background
In modern optical data storage systems, data are stored on an optical storage medium in the form of marks carried on a surface of the optical medium. The data may be accessed by focusing a laser beam onto the data surface of the optical medium and analyzing the light reflected by the marks. Storage density of the system is determined by the size of the beam (called the xe2x80x9cspotxe2x80x9d) focused on the data surface. Consequently, the spot size affects the data storage density of the optical storage medium: the smaller the spot size, the greater the storage density. In addition to optical data storage. applications, reduction of spot size is beneficial for photolithography and microscopy applications as well. For example, in photolithography, smaller spot sizes allow for the exposure of finer features in photoresist.
In each of these applications, knowledge of the spot size is critical to designing an appropriate system. The size of a spot is typically determined by moving an obstruction, such as a ruling or a knife-edge, through the plane where the spot size is to be measured. To accurately measure the spot size using such a technique, the position of the knife-edge or the spacing between the opaque portions of the ruling must be precisely known. However, as spot sizes become smaller, such as on the order of nanometers, it becomes increasingly more difficult to accurately determine the spot size. This is, in part, because movement of the obstruction induces other types of mechanical motions which adversely affect the measuring system. In addition, the position of the knife-edge or the periodic spacing of the ruling must be even more accurately determined.
The problem of measuring focused spot sizes is even further complicated in near-field systems, where it is typically necessary to measure the spot size in a plane which is only a quarter-wavelength (xcex/4) from a lens. This is because it is mechanically very difficult to move the obstruction in the measurement plane when it is so close to the lens.
Accordingly, there exists a need for a technique with which one can accurately measure increasingly smaller spot sizes. There further exists a need for such a technique to be accurate in near-field applications, such as a near-field optical head. There further exists a need for such a mechanism to have minimum susceptibility to mechanically-induced inaccuracies. There further exists a need for such a mechanism to be self-calibrating.
The present invention is directed to a system for measuring a size of a focused spot of an optical energy beam. According to one embodiment, the system includes a grating, a beam scanner oriented between a source of the optical energy beam and the grating, and a detector oriented to sense a diffraction pattern of the grating.
According to another embodiment; the present invention is directed to a system for measuring a size of a focused spot of an optical energy beam, including a grating, means for scanning the focused spot relative to the grating over a scanning range that is greater than a groove width of the grating, and a split detector oriented to sense a diffraction pattern of the grating as the focused spot is scanned relative to the grating.
The present invention provides several advantages in comparison to existing techniques for measuring the size of a focused optical spot. Significantly, the technique of the present invention is self-calibrating and more precise than existing techniques. In addition, the present invention is less susceptible to errors introduced by noise and mechanical vibrations. Also, the present invention permits measurements of the focused spot sizes over a greater measurement range than existing techniques. Furthermore, the present invention may be used to measure the focused optical spot size for both near-field and far-field applications. These and other benefits of the present invention will be apparent from the detailed description hereinbelow.